The conventional printed wiring (PW) circuit board has proven to be an excellent interconnection medium for solid state circuits. Using this approach high reliability at low cost has been achieved for a wide variety of electronic circuit functions. PW circuit boards are ordinarily quite stable and durable, but problems can develop when they are used in harsh ambients. In automotive applications the conditions within the engine compartment are extreme. These include large temperature variations, exposure to corrosive fumes and exposure to moisture. When the equipment is located inside the automobile passenger compartment, the severity of the environment is greatly reduced, but substantial problems still exist. Circuit boards are still subjected to high humidity and corrosive atmosphere conditions along with substantial temperature extremes.
The combination of moisture with corrosive fumes constitutes a particularly severe problem because it can create conductive paths on insulating surfaces. This means that conductive paths exist where none are desired. This is most troublesome in high impedance circuits. Ordinarily solid state circuits operate at low impedance levels and are relatively tolerant of moderate leakage. However, in some applications high impedance circuits are called for and these constitute a special problem.
One solution to the problem is to hermetically seal the circuit board inside a suitable housing. This approach is expensive and vitiates some of the advantages of PW. Another solution is to spray coat the circuit board with a protective film or coating of a suitable plastic. This is helpful but does not completely solve the problem.
In recent years electronically operated automatic speed control devices have been developed and are being used in increasing numbers. These devices employ high impedance circuits which are subject to the potential leakage problem to which the present invention is directed. A typical state of the art speed control system is shown in U.S. Pat. No. 3,455,411, issued July 15, 1969. Speed control is achieved by comparing an analog voltage proportional to speed with a reference voltage. Any departure from a desired difference causes the automobile accelerator to be operated to reduce the departure. This constitutes a servomechanism that acts to hold vehicle speed relatively constant.
Electronically the reference voltage is a charge on a high quality dielectric capacitor. Very little leakage can be tolerated because the reference voltage must be kept constant for extended time periods. The circuit for comparing the reference voltage with the speed analog voltage must not discharge the capacitor and therefore must be a high impedance circuit. By way of example, if the reference voltage is stored in a 0.01 microfarad capacitor and the voltage is not allowed to vary more than 1% over a one hour interval, a time constant interval of over 6,000 seconds is required. For the above capacitor a resistance on the order of 6 .times. 10.sup.11 ohms or 600,000 megohms is required. This high resistance is ordinarily achieved by using an insulated gate type of transistor and special wiring and shielding techniques.
Other portions of the speed control circuit operate at substantially lower resistance values. For example a 30 megohm resistor is employed in the accelerate circuit. Reliable operation of this circuit requires that leakage values of in excess of 3000 megohms be maintained. Such values are quite difficult to maintain on commercially produced products used over long periods of time in the automotive environment. It is to this latter problem that the present invention is directed.